It is common to employ actuators powered by pneumatic signals in order to move the internal trim of modulating type control valves. Such valves are utilized in process plants in order to control the pressure, temperature, or mixture of certain fluids in a closed-loop control system.
It is very important for the stability of such loops that there is very little dead-time, that is any response of a control valve to an up-set in the system should ideally be instantaneous. To that end, it is vital that friction forces either in the actuator, or in the valve itself, are kept to a minimum. The reason is that it takes a certain time for a pneumatic signal to change enough in order to overcome such friction forces. This in turn generated dead-time and causes loop instability.
There is a certain valve type on the market called a sliding gate valve. Such a valve is more closely described in my U.S. Pat. No. 3,955,591.
Such a valve type offers certain advantages such as compactness, low weight, reduced cost, a higher flow capacity, and a lower noise level. The major drawback is that the trim element consist of two slotted plates that have to slide against each other, thereby selectively exposing slots to fluid flow (see drawing FIG. 1). This sliding motion generates a good amount of friction. Present means to overcome such friction range from using piston actuators utilizing high air pressure, or, to use oversized pneumatic diaphragm actuators. Either of these solutions negate a good part of the cost advantage of the valve itself. One other problem with large actuators is that they are designed for relatively large travels. Yet, the travel requirements of sliding gate valves are very small, hardly exceeding the width of a slot within the sliding plate. The necessary use of only a small fraction of the available actuator travel again leads to a loss of positioning accuracy of the complete valve system.
There is therefore a need for a small and compact pneumatic actuator that has a high output force of a heretofore much larger actuator in order to overcome the aforementioned friction problem.
The present invention overcomes these problems by providing a small and compact actuating device that is able to amplify the force generated by an air signal acting on a diaphragm by typically three times, while at the same time reducing the normal travel of the diaphragm by a like ratio, thus meeting the exact requirements of such sliding gate valves.
Internal force amplification for valves and valve actuating devices have been used before. Typical devices are described in my U.S. Pat. Nos. 4,609,178 and 4,684,103.
U.S. Pat. No. 4,609,178 for example describes means to amplify the force of a valve stem within the valve itself. Here a wedge is used to drive a pair of ball bearings apart leading to a like motion of a second pair of bearings mounted on fulcrumed linkages. Such a system would not be workable for my intended purposes since the wedge exerts only a unilateral force to overcome reaction forces by fluid pressures acting on a diaphragm. In addition, there is no space within a sliding gate valve to incorporate an amplifying device as described herein. My U.S. Pat. No. 4,684,103 is very similar in nature. U.S. Pat. No. 4,684,103 describes again a bearing and fulcrum linkage device opposing fluid forces acting on a diaphragm. Here the actuator and valve are combined, a method not suitable for my purpose. Note also the absence of a secondary coiled spring providing sufficient force to oppose the force created by a pneumatic signal acting on a flexible diaphragm taught in my invention.
My invention does provide for a separate and dismountable actuating device with internal amplifying mechanism that is not only compact and of low cost, but one that also provides ample force amplification at a high degree of mechanical efficiency, as can be seen from the following description.